High-voltage interactions in plasma wakes: Results from the charging hazards and wake studies (CHAWS) flight experiments

Data from the charging hazards and wake studies (CHAWS) flight experiments on board space shuttle missions STS‐60 and STS‐69, during which a negatively biased, high‐voltage (0–5 kV) probe was placed in a plasma wake in low Earth orbit, are presented. For these experiments the source of the wake was the 4‐m‐diameter Wake Shield Facility (WSF), which was operated both as a free‐flying spacecraft and attached to the shuttle orbiter's robot arm. Current collection by the biased probe is investigated as a function of the density and temperature of the ambient plasma and the probe's location in the plasma wake. Current collection behavior is determined by the expansion of the high‐voltage sheath into the ambient plasma stream. Consistent with preflight predictions, current collection on the probe is highly nonuniform, varying by more than 5 orders of magnitude across the surface of the probe. The onset of current collection, however, begins at voltages that are an order of magnitude lower than anticipated. This is likely due to the low‐energy, turbulent plasma (typically 2–5% of the ambient density and up to 40% on occasion) observed in the ambient environment. This important minority constituent of the plasma was observed in the vicinity of the shuttle Orbiter and observed while the WSF was free‐flying.